Thermionic valve amplifier



'June 1, 1937. N. M. RUST ET AL 2,082,517

THERMIONIC VALVE AMPLIFIER Filed June 7, 1935 INVENTORS NOE L M RUST FRANCI MURPHY BY AT'ToRNEY Patented June 1, 1937 UNITED STATES PATENT OFFICE 2,082,517 THERMIONIC VALVE AMPLIFIER of Delaware Application June 7, 1935, Serial No. 25,414

V In Great Britain May 28, 1934 2 Claims. (01. 179-171) This invention relates to thermionic valve amplifiers, and more specifically to tuned high frequency thermionic valve amplifiers.

The invention has forits object to provide an 5 improved tuned amplifier by which high gain and good selectivity can be obtained.

More specifically the invention has for its object to avoid the limiting eifects of a difficulty to be described later herein and which hitherto has prevented the obtaining of as high a selectivity and as high an amplification as might be desired for'multi-stage thermionic valve amplifiers, such as tuned radio frequency amplifiers.

In order that the invention may be better understood certain relations existing in a radio frequency thermionic valve amplifier stage will first be described. If the valve in such a stage has an external anode impedance, that is to say a load impedance Z0; an input impedance (control grid-cathode) Zg as shown in Fig. 1; an anode impedance i. e. internal impedance, R and an anode-control grid capacity Cgp of reactance Xe it may be shown that This is quite commonly the case with screen grid valves for in such valves ,4 is usually high 45 and Z0 will also be high if good quality inductance coils are used with the ordinary'values of tuning condenser common in radio receivers.

It will also be clear that Zg will generally be lowest at the highest frequencies since 50 is inversely proportional to frequency. If Z0 is a resistive load, as it is when the anode circuit 55 ofthe valve is intune, vZ will be equivalent to a resistance shunted by a condenser and the equivalent condenser will detune the grid circuit of the said valve, while the equivalent resistance will increase its damping so that the selectivity of the whole stage is impaired. If, however, Z0 5 is an inductive load (as it may quite easily be if the stage is one of two stages which are unicontrolled for tuning purposes and the gauging is not quite accurate) Z may become equivalent to a negative resistance in parallel with a coni0 denser and if this negative resistance is lower than the dynamic resistance of the grid circuit, the stage as a whole will become unstable.

The effect described above and sometimes known as the Miller eifect, presents a serious 15 difficulty in high frequency thermionic valve amplifiers, and more particularly in cascade connected screen grid valves and similar amplifiers where valves feed into load impedances of high value and are of high amplification factors. 20

According to this invention in its broadest aspect a high frequency valve amplifier having a tuned grid circuit has its cathode-grid space tapped down upon said' tuned circuit, i. e. in effect connected across only a part thereof. a I

Inthis way, by suitably choosing the tap down ratio (1. e. the ratio between that portion of the tuned circuit connected between cathode and grid and the whole tuned circuit) it is possible to make the effect upon the circuit of the impedance Zg practically negligible, and thus to avoid the deleterious effects due to capacity thrown back from the anode to the grid, and accordingly obtain an increase both in stability and selectivity. In carrying out this invention the tapping down is efiectedcapacitatively, i. e. by connecting the grid-cathode circuit across a tap down condenser which is in series with another condenser in the tuned circuit (thesetwo condensers replacing the usual single condenser of the tuned circuit) and, in general the impedance of this tap down condenser (which is eifectively in parallel with the impedance Zg) must, at the frequency to which the circuit is tuned, be considerably smaller in value than the impedance Zg, to secure the correct tap down condition. The ratio of these two impedances is preferably kept constant at all operating frequencies.

In the drawing:

Fig. 1 shows an amplifier circuit employing the invention, v Figs. 2, 3, 4 show respectively dilferent modifications of the circuit in Fig. l, and v.

circuit is constituted by an inductance (IL; 2L)

shunted by two condensers (ICI, IC 2; ZCI, 202) in series and the control grid of each valve is connected to a point between the two series con: densers. Of these two series condensers that (I02: 202) which is nearer the cathode side of the tuned circuit is large relative to the main tuning condenser, which is the condenser in series therewith. In many cases, owing to the fact that the impedance of the largercondenser falls as the frequency rises, a sufliciently close approximation to the theoretically required con- ,ditions can be obtained by making this large confdenser of fixed value, but inother cases it may be desirable to make this large condenser (I02;

2C2) variable-and to gang control it with the main tuning condenser (ICI: '20!) and, if desired, with condensers in the other tuned circuits of'the'whole amplifier. CRI and CR2 represents the ,usual capacity shunted resistance combinations for self-biasing; A represents an aerial or other source of signals to be amplified; and +SG and +HT represent screen grid and anode potential supply terminals respectively. The impedance Z is shown in dotted lines in Fig. 1, and represents the cathode to control grid impedance of amplifier ilV. The representation is not shown in connection with the remaining amplifiers of thedrawing, but it will be understood that it exists in such amplifiers in the same manner as shown in.connection with'amplifier IV.

} In the modification shown in Fig. 2, the arrangement of .Fig. lis modified by replacing the members 102 and 202 by series tuned circuits (IST; 2ST) eachresonant at a frequency above the" tuning range of the amplifier so as always to present capacitative impedances, while in the modification shown in Fig. 3 parallel tuned circuits (lPT; 2PT) 'are employed, these circuits being resonant at a frequency below the tuning range so that again they always present capacitative impedances. In both Figs. 2 and 3 the ftuned circuits may be fixed or variable and in the latter case may be gang controlled, but in any case their tuning must always be such, relative to the frequency to be amplified, that their impedances are capacitative.

' The feature of tapping down the grid upon what is in effect a capacity potentiometer, may be embodied in cases in which the tuned circuits are tuned by variation of inductance instead of, 'or jas'well "as, variationpf capacity and Fig.4 shows 'anembodiment where variable inductance tuning'is' used. As in'the previous embodiments gang-control may be'provided if desired and suchcontrol is represented in Fig. 4 by the broken line GL. 7 g

The invention is also applicable'toaflangements wherein, in place of using simple tuned circuits, band pass tunable circuit arrangements are employed. For example in one such case illustrated in Fig. 5 the control grid GI of a 'screen grid valve is' tapped :upon' a band pass tuned circuit consisting of two variableinductances ..lLI,-.IL2, a variable condenser ICI', a larger fixed condenser |C2,"a second similar fixed condenser I62" and a second variable condenser ICI" all in series in a loop circuit in the order stated, may be employed, the two variable condensers being similar and the junction point of the two inductances being connected to the junction point of the two large fixed condensers through a further fixed condenser CO. The two variable inductances and the two variable condensers are gang controlled for tuning purposes 11 as indicated by the broken line GL and may, if

, inductance lLl adjacent thereto is connected to the anode of a preceding valve (not shown) or I to 'some other source of signals to be amplified. Where-thesaid source is the anode of a preceding valve, anode potential may be appliedthereto as shown from a 'point HT+ through a load connected to the junction point of the two variable inductances ILI and IL2.

It is found that the invention enables high selectivity and high gain to be obtained in multistage radio frequency tuned amplifiers, and is, therefore,.of great advantage in broadcast re ceivers, the feature of tapping down providing a very substantial practical improvement. It is to be understood that, particularly where the invention is applied to very high frequency multi-stage amplifiers, careful screening and decoupling arrangements, as known per se, should be provided to prevent undesired stray couplings between various parts of the amplifier.

It is to be understood that although in the illustrated embodiments four-electrode screen grid valves areshown, the invention is not limited to the use of this type of valve, for pentodes, triodes, or other suitable types of amplifying valve may be employed. a

What is claimed is:

1. A tunable high frequency amplifier circuit comprising an electron'discharge tube of the screen grid type, a source of high frequency signals to be amplified, a signal input network coupling said source ftothe input electrodes of the tube, said input network comprising a coil in shunt with a path including at least two capacitative impedances in series, the tube input electrodes being connected solely across one of the impedances to substantially minimize the effect of the impedance between the input electrodes upon the input network, the other impedance being smaller in capacity value than said one impedance and being adjustable to vary the tuning of the amplifier over a :desired frequency range, the magnitude of saidone impedance being substantially smaller than that of said input impedance at any frequency to which said input network is tuned, andan output circuit coupled to the output electrodesof the tube.

trodes being connected solely across oneof the impedances to substantially minimize the effect of the impedance between the input electrodes upon the input network, the other impedance being smaller in capacity value than said one impedance and being adjustable to vary the tuning of the amplifier over a desired frequency range, the magnitude of said one impedance being substantially smaller than that of said input impedance at any frequency to which said input 

